In the case of known methods a sample of granules is taken from the product stream generated and in a subsequent step is processed so as to yield a molding which is subjected to the actual color measurement with the aid of spectroscopic methods. This procedure entails a considerable delay between the production of the colored granules and the quality-assuring color test on the molding. If deviations in the color do occur, considerable quantities of faulty batches are produced before the deviation in color can be recognized and suitable countermeasures can be taken. If production is enabled only after a molding specimen has been tested, this will result in machine downtimes which are uneconomical.
This is why in-line measuring methods had indeed been developed earlier, which indicate the hue without any delay. Thus EP 0 407 927 previously described a method for the continuous measurement of the hue of a colored plastics molding compound, involving the characterization of plastics granules in terms of their hue when they pass a measuring window. This method suffers from the drawback of the concomitant reflections. There is also the possibility of the measurement result being affected by moisture deriving from the granulating process. U.S. Pat. No. 5,369,483 further describes a method for measuring the hue of a plastics molding compound, wherein the scattered light of a plastics melt is analyzed by means of a glass fiber bundle. This method suffers from the drawback of the low long-term stability of commercial fiber bundles. Moreover, the geometry of the parts in contact with the product is unfavorable in principle, since the generally viscous melts will result only in low exchange, of product at the measuring surface. Another adverse factor is that the illuminated surface is within the range visible to the detector and that consequently minute, unavoidable contamination may lead to considerable measurement errors. EP 072 899, finally, discloses a photometric measuring device which employs a backscattering technique. However, the results achieved here are not optimal in terms of the specific applications aimed for in the present invention.